ps418

There are two fairly new books reviewing the evolution of birds. I highly recommend both, in light of the fact that quantity and quality of fossil evidence has exploded beyond anyone's expectations in the past 5 years or so. Particularly interesting are the chapters in Chiappe and Witmer reviewing Mesozoic birds from China and Mesozoic feathers.

Mesozoic Birds: Above the Heads of Dinosaurs by Luis M. Chiappe (Editor), Lawrence M. Witmer (Editor)

Info from UC Press

And

Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds, by Gregory S. Paul
Review of Paul's book

Patrick

MrDarwin

Gregory Paul is the guy who thinks that at least some of the theropod dinosaurs were descended from birds, rather than vice-versa. It's an interesting theory and I do find myself wondering why some of the "feathered dinosaurs" are classified as such rather than as secondarily flightless birds. (Maybe that explains the mystery of why Tyrannosaurs had such ridiculously small "arms"--perhaps they were actually vestigial wings somewhat like those of modern ratites. Heck, maybe they still had feathers on them and were used for courtship displays!)

Part of the problem is that early birds were so dinosaur-like that if you strip off their feathers, you basically have a dinosaur. Even their "wings" are nearly indistinguishable from dinosaurian forelimbs, except for the feathers--in morphology they are certainly nothing like the wings of modern birds (whatever Ed may claim!). At some point it's pretty much arbitrary whether you call a fossil a bird or a dinosaur. These early birds are just about the coolest fossils that have ever been found and it's been hard to keep up with the literature.

ps418

I dont think there is any compelling reason IMHO to think that, for instance, Caudipteryx and Protarchaeopteryx, are secondarily flightless, as opposed to being morphologically close to the stem of Aves. In chapter 1, pp. 11-14, of Chiappe and Witmer, Witmer argues in great detail that Caudipteryx is not a secondarily-flightless bird, and shows that it retains numerous primitive characters that are already lost in Archaeopteryx. They also discuss Paul's arguments, finding them wanting. This is significant, because Caudipteryx clearly has "avian" feathers, with calamus, rachis, and barbs, as opposed to the less feather-like fuzz on some other closely related dinos.

Witmer concludes that the "picture that emerges from this brief survey of Caudipteryx is of a feathered theropod dinosaur that is probably well outside of the avian lineage. . . it seems readily apparent that it would be much less parsimonious to include Caudipteryx within aves" (p. 13). Cladistic analyses, for instance that of Sereno (1999), also place Caudipteryx outside of aves. But whether it is placed in aves or not, it remains fanatastic evidence for the theropod ancestry of birds. Witmer again: "forcing Caudipteryx to be within aves because of its possession of true feathers would not automatically strip it of its clear theropod heritage" (p. 14).

Indeed. There is extensive morphological 'intergradation' between therapoda and aves. However, in my opinion it is well-supported that bonafide feathers occur in nonavian dinosaurs, particularly Caudipteryx. Witmer again: "Feathers -- that quintessentially avian trait-- have always been the great definer of birds. The presence of unambiguous feathers in an unambiguously nonavian theropod has the rhetorical impact of an atomic bomb, rendering any doubt about the theropod relationships of birds ludicrous" (p. 14).

And the proof of this is that the characters that define aves have become so trivial. It used to be easy. Birds had wishbones, feathers, retroverted hallux, and a retroverted pubis. Now we know that each of these features, with the possible exception of hte retroverted hallux, are found in nonavian theropods. Now aves is defined by characters like elongate prenarial portion of the premaxilla and other extremely subtle characters. Witmer notes that "Some of the foregoing characters may seem a bit subtle, even trifling, but such is the nature of of any phylogenetic transition as it becomes better and better known. In fact, we should predict that the number of characters per node should decrease as (and that the characters themselves may well seeem more trivial) as sampling of the fossil record improves" (p. 7).

Too true.

Patrick